Due to the enchanced precision with which ophthalmic surgery can now be performed using laser instruments, it is increasingly important that corneal topography be determined with an accuracy which will allow full exploitation of the capabilities and potential of these instruments. Further, it is important and desirable that the entire anterior surface area of the cornea, and not just a part thereof, be accurately mapped in order for laser instruments to be employed most efficaciously. To this end, several devices have been proposed.
One type of instrument which is now used to measure the anterior surface of the cornea relies on the use of Placido disc illumination. In accordance with the technique used by such an instrument, a template having a series of expanding planar concentric circles, or rings, is positioned in front of the eye and the light reflected from the cornea through the template is analyzed. Specifically, the distortion of light reflected through the ring pattern by the conditions that exist on the corneal surface is analyzed to help determine the corneal topography. As can be expected, the efficacy of this technique is dependent on the precise geometric placement of the ring pattern with respect to the corneal reflections. Importantly, however, the accuracy of this technique is greatly diminished, indeed obviated, as the diameter of the ring is diminished. Unfortunately, this occurs in the center of the template which receives light reflected from the center of the cornea. Thus, the diminution in accuracy accurs at the very part of the cornea where the greatest accuracy is required.
Another type of device for analyzing a corneal contour employs a modal wave front estimation from phase derivative measurements. An example of such a device is disclosed and discussed in an article written by Philip C. Baker entitled "Holographic Contour Analysis of the Cornea" which appeared in SPIE Vol 1161 (1989). In the operation of this device, a wave front reflected from the corneal surface is detected and analyzed to determine phase change interferences which result from perturbations on the surface. An interference fringe pattern is thus created with data which can be digitized for evaluation. There are, however, some disadvantages with such a device. For example, the return signal is extremely noisy. Thus, there is a high probability for random reconstruction error. Further, the technique involves numerical complexities and an inherent problem for determining compatible polynomials. Still further, and perhaps most importantly, phase derivative measurements require analysis of an unperturbed reflected wave front. Stated differently, the signal to be analyzed is spread over the whole wave front. Consequently, in order to obtain a signal which will yield a fringe pattern with sufficient information to be meaningfully analyzed, the light source used must provide a relatively high intensity input, i.e. a flash of light. This, at best, can be uncomfortable for the patient.
In light of the above, an object of the present invention is to provide a device for mapping the topography of the eye which provides a uniform homogeneous mapping of the entire corneal surface of most significant interest. Another object of the present invention is to provide a device for mapping the topography of the eye which is equally accurate for all portions of the anterior surface of the cornea which are of significant interest. Still another object of the present invention is to provide a device for mapping the topography of the eye which gives information that can be beneficially used for a wide variety of ophthalmic procedures. Yet another object of the present invention is to provide a device for mapping the topography of the eye which is relatively easy to use and comparatively cost-effective.